In recent years there has been growing interest in the use of organic semiconductors, including conjugated polymers, for various electronic applications.
One particular area of importance is the field of organic photo voltaics (OPV). Organic semiconductors (OSCs) have found use in OPV as they allow devices to be manufactured by solution-processing techniques such as spin casting and printing. Solution processing can be carried out cheaper and on a larger scale compared to the evaporative techniques used to make inorganic thin film devices. State-of-the-art OPV cells consist of a blend film of a conjugated polymer and a fullerene derivative. Recent improvements in the efficiencies of single-junction OPVs, Yu et al., Nat. Photonics 2014, 8, 716. (efficiency ˜8-9%), have largely been due to the development of low-band-gap polymers, which are defined as polymers with an absorption onset of at least 750 nm or more and with a band-gap of 1.65 eV or less. (For example, a low-performance OPV polymer, P3HT, (bandgap ˜1.9 eV) is not considered the state-of-the-art polymers for OPVs.)
The polymers commonly used in PSCs consist of an electron donating (donor or D) and an electron accepting (acceptor or A) comonomer units. It is convenient to use such a D-A alternating copolymer strategy to obtain polymers with low optical bandgaps as the HOMO level of the polymer is mostly located on the donor unit and the LUMO level mostly on the acceptor unit. The commonly accepted model developed by Brabec, etc. indicates that elaborately designed HOMO and LUMO energy level is a basic requirement for high-performance polymer solar cell because open-circuit voltage (Voc) of polymer solar cells is determined by the difference between the HOMO level of the polymer and the LUMO level of the fullerene derivative. LUMO energy level is relatively more important because LUMO offset between polymer and fullerene should be small enough to minimize Voc loss. By modifying the acceptor unit with electron-donating or withdrawing groups, the LUMO level of the D-A polymer can be effectively tuned, while the same can be done to tune the HOMO level by modifying the donor unit.
In the area of conjugated polymers for PSCs, fluorination method has been used to modify conjugated polymers to tune the HOMO and LUMO levels. There are several reports on fluorinating the acceptor unit of the D-A conjugated polymers, which turns out to be an effective method to adjust the energy levels of conjugated polymers accompanied by other positive effects including enhanced polymer stacking ability and crystallinity. For example, You et al., J. Am. Chem. Soc. 2013, 135, 1806, reported that difluorination of benzothiadizole unit lead to lower HOMO level and thus enhanced Voc of the solar cell as well as increased polymer stacking in the solid state. In contrast, there were few successful attempts in fluorinating the donor part of D-A conjugated polymers, which has yielded improved performance for the PSC devices. It was believed in one case that a perfluorinated polymer backbone (with fluorine on both the donor and accept units) lead to poor PSC performance attribute to too strong self-organization property and fluorophobicity effect of the polymer.